Polychloropyridines are well-known to be useful intermediates in the preparation of agricultural chemicals, pharmaceuticals and antimicrobial compounds. 2,6-dichloropyridine is one such intermediate, and there are several known processes for making this intermediate from 2-chloropyridine. In recent years, there has been a trend in the industry away from the use of catalysts in preparing the 2,6-dichloropyridine since catalysts typically require additional expense, as well as processing costs associated with catalyst removal from the desired product. Illustrative of this trend, U.S. Pat. No. 5,112,982 discloses a process for preparing 2,6-dichloropyridine by reacting 2-chloropyridine with chlorine in a liquid phase at a temperature not lower than 160.degree. C. in the absence of a catalyst. Unfortunately, the reaction of the process of the '982 patent takes longer than might otherwise be desired, as shown by the working examples of the '982 patent which involve reaction times ranging between 40 hours (Example 2 of the patent) and 50 hours (Example 1 of the patent).
Other processes which typically do not require the use of catalysts are also known. By way of illustration, U.S. Pat. No. 3,557,124 discloses a process for the production of 2,6-dichloropyridine in a liquid phase reaction at a temperature within a range of from 90.degree. C. to 180.degree. C. Unfortunately, the product yields obtained using the process of the '124 patent are sometimes lower than might be desired, as shown in the first run of Example 1 (72.3 percent yield) and in Example 4 (76 percent yield) of this patent.
As still another illustration, U.S. Pat. No. 3,251,848 discloses a vapor phase process for producing 2,6-dichloropyridine which comprises rapidly mixing, in a turbulent flow system and a preferred temperature range of from about 370.degree. C. to about 395.degree. C. Unfortunately, this preferred temperature range is considerably higher than might be desired from a processing cost standpoint.
Illustrative of other processes for preparing polychloropyridines are the following: U.S. Pat. No. 3,186,994 discloses a process employing polychloro-(trichloromethyl) pyridine as a starting material; and, U.S. Pat. No. 4,701,532 discloses the selective chlorination of 2-chloro-5-(trichloromethyl) pyridine in a liquid phase reaction using an effective amount of Cl.sub.2 at an elevated pressure and elevated temperature in the range of 60.degree. to 180.degree. C. in the absence of an added catalyst. However, these processes generally produce product mixtures, and the selectivity to, and purity of, the desired specific product typically is lower than might be wanted.
Another noteworthy trend in recent years has been away from the use of environmentally unfriendly solvents, such as carbon tetrachloride. However, heretofore, organic solvent-free processes have generally required higher temperatures than might be desired.
New processes for producing 2,6-dichloropyridine that are simple, do not employ a catalyst and do not employ an organic solvent, and yet provide good selectivity to a highly pure desired product without significant amounts of byproduct production, would be desired by the 2,6-dichloropyridine manufacturing community. The present invention provides one such process.